This invention relates to blind fasteners for use in fastening two panels together in overlapping outer and inner relation and, more particularly, to a blind fastener for fastening panels made of composite or other relatively soft materials.
Blind fasteners are used in a variety of applications in which access to the blind side surface of panels being connected together is extremely limited or in some cases not possible. For example, in the construction of aerodynamic designs, including aircraft and the like, a substantially flush surface usually is desired on the accessible side of the panels, while access to the blind side may not be possible. Such aerospace fasteners must meet or exceed stringent requirements. In particular, the fasteners must securely fasten the panels and resist losing their gripping power under the stresses, vibrations, and temperatures imposed upon them by the harsh environment in which they are used.
One type of blind fastener that satisfies these requirements comprises an internally threaded fastener body for insertion into aligned holes of the two panels, and an externally threaded cylindrical stem passing in threaded engagement through the fastener body. The inserted end of the stem has an enlarged stem head, and the outer end of the stem has a wrench engaging portion. Upon turning motion of the stem relative to the fastener body, the stem is moved in an axial outward direction through the fastener body. This axial outward movement causes a deformable sleeve around the stem and abutting against the stem head to deform around a tapered nose on the fastener body to a fully set condition against the inner panel. The stem further may be provided with a localized weakened region or break groove adapted to shear the stem at a predetermined torque. The break groove is preferably located axially along the stem such that the stem twists off in a substantially flush relation to the outer portion of the fastener body, i.e., the fastener body head, after the fastener is fully set. The fastener body head normally is received in a countersunk, flush relationship to the outer panel, thus providing an aerodynamic surface after the fastener is set.
Special considerations arise when blind fasteners are used to secure panels made of composite or other relatively soft or lightweight materials. When the deformable sleeve of a blind fastener reaches its fully set condition, it applies very high and localized stresses against the inner panel. Panels made from composite materials normally are unable to withstand these high localized stresses. As a result, the panels usually are crushed and damaged. To help alleviate these problems, when fastening panels made of composite materials, a deformable sleeve constructed from relatively soft material is used. In addition, the deformable sleeve usually is configured so that it will bulge laterally when being set against the inner panel to provide a large bearing area and distribute compression forces over a wider surface area of the inner panel.
In general, blind fasteners having the foregoing type of deformable sleeve provide a satisfactory area of engagement with the blind side of the inner panel, even though the deformable sleeve is made from a softer and, therefore, weaker material than blind fasteners used to set panels made from steel or other high strength materials. However, in some applications with stress sensitive panel materials, such as composite materials, a high strength deformable sleeve is desired, yet it cannot be used since it will tend to crush or damage the panel material.
Accordingly, there has existed a definite need for a blind fastener having a deformable sleeve that provides a large area of engagement with the blind side of the inner panel, yet provides the strength advantages of a deformable sleeve made from a harder material. The present invention satisfies this need and provides further related advantages.